In some applications, a process designer will have to decide if a given function should be run with open- or closed-loop control. There is a tendency to think more about closed-loop applications where the manipulated variable is adjusted automatically to maintain a setpoint. On the other hand, open-loop control is done manually. An operator sets a valve or other actuator at a given point and it stays there until it is changed manually again. This could be a situation where an automatic loop is placed in manual. While the differences between the two options sounds clear, sometimes it can be complicated.

For example, a chemical/process engineer was walking at a refinery’s tank farm in Indonesia, which was separated physically from the refinery, He came upon an odd-looking tank equipped with only a crude site glass level indicator, and a man sitting on a stool next to a hand-valve on a line exiting the tank. The man was in charge of monitoring the site glass and moving the valve manually as necessary. Is this open- or closed-loop control?

All the elements are there: an input from a level transmitter, an output to a control valve connected by a pneumatic or electronic logic solver in the middle, and a worker replacing all three functions in real-time. The unique characteristic of this control loop that makes it closed-loop is the absence of a significant time delay between the sensing of a change in the dependent (controlled) variable, and a move in the independent (manipulated) variable. The operator can respond very quickly to a change in the sight glass with a change to the valve position.

Most closed-loop controls do suffer from some inherent lag and deadtime associated with either the measurement of the controlled variable, or the response of the controlled variable to a change in the manipulated variable. However, the essential point of closed-loop control is that the response to the sensing of the change in the controlled variable is close to instantaneous.

The quality of control of a loop with extensive dead time or lag can be measured. One approach examines the standard deviation of the controlled variable over a long period. It will degrade in proportion to the amount of dead time. If the operator at the Indonesian tank farm isn’t constantly watching the tank level and operating the valve, how closely will the process variable track its target?

Open-loop control

Many refineries and chemical plants use optimizers to determine targets for key operating variables, such as charge rate, product specifications, reactor severity, etc. Optimizers require external inputs, such as feed and product prices, supply-and-demand figures, fuel costs, etc., to calculate optimum operating targets. How do such external inputs get into the optimizer, and how do the optimizer-generated targets get loaded into the control system?

External inputs usually are entered manually by designated personnel in the company’s planning department. Resulting key operating targets typically are communicated to the control room via an email or spreadsheet-but not closed loop-and only after someone in the planning department has reviewed the optimizer solution to verify its reasonableness and feasibility. The control room operator enters the key operating setpoints to "optimize" the unit.

Since setting optimizers in refineries and chemical plants still uses open-loop control, the external variables may change, but the process will continue running where it was set. The optimizer solution that was implemented Monday is probably using older feed and product prices from Friday because no one was onsite since Friday to manually change it. Prices may have changed significantly since Friday, so once implemented, refinery operation may be far from where it should be for Monday’s market conditions. Hopefully by Tuesday, the planning department will catch up.

The key point is that the logic solver, be it a human being, a proportional-integral-derivative (PID) controller, or a refinery optimizer, must be making decisions in real-time to provide the best control performance. Otherwise, it’s open-loop controls, may not be performing optimally. That’s the key difference between open- and closed-loop control. Control is better with closed-loop, and when the time between changes in the process, variable and automated corrective action is kept to a minimum.